Although fiber-based cigarette filter elements are well known, the choice of components for such filters has remained quite limited, over the years, because of cost factors and the lack of suitability of many natural fibers for high speed filter production, using state of the art filter rod-making apparatus. In addition, the functional requirements of modern cigarette filter elements tend to conflict with respect to filtration efficiency and selective filtration of cigarette smoke. In particular it is desired to pass along flavor while limiting delivery of less desired smoke by-products.
While various synthetic fibers and fiber mixtures have been tried and evaluated as filter components, a substantial number of modern cigarette filter elements continue to use old technology and well known substrates because of cost and handling advantages. For example, cellulose acetate tow using a variety of additives can be processed into cuttable filter rods using an essentially unmodified state-of-the-art filter rod-making apparatus without serious jamming problems. This advantage is useful in view of the increased need for additives, including organic plasticizers such as triacetin and diacetin, as well as lubricants, flavors, medicines, and particularly selective filtering additives. Generally, such compositions are directly applied onto fiber tow in the form of aqueous solutions or suspensions by using art-recognized dipping, spraying, or printing techniques.
The advantages of cellulose acetate fiber as filter substrate, however, are countered by serious disadvantages. For example, such fibers tend to be relatively weak, compared with synthetic thermoplastic fiber such as polyolefin fiber. This characteristic limits the amount of tension and crimp that a fiber tow of low dpf fiber or filament will tolerate before introduction into a filter rod-making apparatus. This, in turn, can limit the structural integrity or hardness of the filter.
Synthetic thermoplastic fiber components, particularly polyolefins such as polypropylene staple, when used alone or in combination with cellulose, offer an alternative since they are easily drawn to smaller denier and thereby, offer a potentially high general filter efficiency without serious loss in strength and the above-noted crimping problems and tension under high speed production conditions.
Such artificial thermoplastic fibers, however, also have disadvantages. These generally arise from the fact that such candidates, particularly polyolefins, are hydrophobic and tend to be chemically inert, while a majority of known cigarette filter additives, as above noted, are hydrophilic and difficult to retain in proper amount and functional condition within filter elements containing hydrophobic synthetic fiber as a major component.
Another substantial problem, unique to the cigarette filter art, concerns the difficulty in optimizing fiber denier and general filter efficiency of synthetic fiber filters without corresponding sacrifice in dimensional stability, hardness, and draw (pressure drop) properties across the filter element.
These technical problems are on-going and interrelated, such that attempted improvements in one area usually require difficult choices in others.
It is an object of the present invention to obtain improved cigarette filter elements having superior selective filter retention and pass through properties without sacrificing dimensional stability or general filter efficiency.
It is a further object of the present invention to obtain cigarette filters having improved selective aldehyde filter retention control coupled with significantly increased filter pass through or delivery of flavor components such as nicotine.